1. Field of the Invention
The present invention relates to a digital video signal processing apparatus. More particularly, the present invention relates to a digital video signal processing apparatus and method for compensating a chrominance signal output from 3-dimensional (3D), Y/C separation in NTSC/PAL (National Television System Committee/Phase Alternation by Line) systems.
2. Description of the Related Art
The display of an NTSC/PAL broadcasting system includes a device for processing a CVBS (Composite Video Blanking Sync) signal, which is a composite of a Y (luminance) signal and a C (chrominance) signal. The C signal is quadrature-amplitude-modulated with a subcarrier frequency fsc. Thus, characteristics of the C signal are determined by its frequency and phase. A digital video signal processing apparatus at a receiving stage separates Y and C signals with reference to the characteristics of the C signal and displays an image based on the separated signals.
FIG. 1 illustrates a block diagram of a conventional video signal processing apparatus 100. Referring to FIG. 1, the video signal processing apparatus 100 includes a comb filter 110, a one-dimensional band pass filter (1D-BPF) 120, a weight decision unit 130, a combiner 140 and a subtracter 150. The comb filter 110 one-dimensionally band-pass-filters an input video signal in the vertical direction. The 1D-BPF 120 one-dimensionally band-pass-filters the input video signal in the horizontal direction. The weight decision unit 130 decides weights of the output of the comb filter 110 and the 1D-BPF 120 with reference to vertical/horizontal correlation and the phase of a C signal. The combiner 140 combines the output signals of the comb filter 110 and 1D-BPF 120, in response to the weights, to output the C signal. The subtracter 150 subtracts the C signal from the input CVBS signal to obtain the Y signal.
FIG. 2 is a block diagram of another conventional video signal processing apparatus 200. Referring to FIG. 2, the video signal processing apparatus 200 includes a 2D-BPF 210, a subtracter 220 and a post-processor 230. The 2D-BPF 210 carries out two-dimensional convolution for extracting a modulated C signal. The extracted C signal and a Y signal output from the subtracter 220 are processed by the post-processor 230. When the 2D-BPF 210 carries out incorrect Y/C separation, the post-processor 230 compensates for this to output a compensated Y/C signal.
In conventional Y/C separation techniques, Y/C separation is performed based on comb filtering when the edge of a detected image has high vertical correlation, but Y/C separation is carried out based on 1D band pass filtering when the detected edge has high horizontal correlation. According to the conventional Y/C separation techniques that select one of the filtering methods, as described above, system performance largely depends on a threshold used for edge detection. That is, Y/C separation can be erroneous or unstable if the wrong filtering method is selected due to inaccuracy of edge detection. Conventional techniques can solve the problem to some extent by not selecting just one of the filtering methods, but instead combining the results of the two filtering operations. However, these techniques are based on horizontal or vertical one-dimensional filtering, and usually cause artifacts when the edge direction is not fixed.
In other words, in the case where the direction of the edge of a general image is not fixed, cross-luma and cross-color can appear on a displayed image when Y/C separation is not properly performed. The cross-luma appears as dotted artifacts generated when a C component exists in the separated Y signal, and the cross-color appears as artifacts in a rainbow pattern generated when a Y component exists in the separated C signal.
To improve spatial filtering, spatio-temporal filtering may be used for a conventional video signal processing apparatus. In this case, the current pixel data is processed using the correlations of pixel data of the previous field and pixel data of the next field with the current pixel, or of the previous frame and the next frame with the current pixel. The spatio-temporal filtering method requires a memory for storing the data of the previous and next fields or frames. Although the spatio-temporal filter is more expensive than the spatial filter, the spatio-temporal filtering technique is frequently used when high quality images are required.
However, even when using the spatio-temporal filtering, there still may be some overlap between the Y and C signals in time and/or space in images that have high frequency components in time and/or space, e.g., moving images. This is a particular problem when a portion of the Y signal is present in the C signal, which may result in cross-color. Human vision is very sensitive to the presence of such cross-color artifacts.